The present invention relates to a three dimensional imaging device, that is to say a device for obtaining three dimensional data of a target surface, whether such data is displayed in three dimensional form or not. Indeed, the data may never be displayed as such, but may merely be used to control other equipment. Such an imaging device is useful for supplying three dimensional data to other instruments. For example, such data is valuable in the science of robotics, where objects are required to be identified on the basis of their three dimensional shape, and to be manipulated accordingly. Such data is also useful in monitoring the accuracy of the shape of a series of articles intended to be identical with each other. A three dimensional imaging device of this type is inexpensive to manufacture, high speed in operation, compact, and robust, and hence especially well adapted for use in robotics, e.g. for mounting on the end of a robot arm, although the utility of the present invention is by no means limited to robotics.
Such imaging devices are disclosed in my earlier U.S. Pat. No. 4,645,347 issued Feb. 24, 1987 (the contents of which are incorporated herein by reference) and the equivalent Canadian patent application Ser. No. 505,166 filed Mar. 26, 1986.
More specifically, my prior patent discloses an imaging device that has a converging lens system defining an optical axis, i.e. direction Z, together with a position sensitive light detector that has a series of pixels extending in at least one direction X perpendicular to such Z direction. The detector is preferably bidimensional, i.e. has an array of pixels extending in mutually perpendicular directions, X and Y, both perpendicular to the Z direction.
The lens system serves to simultaneously image a plurality of distributed points on a target surface onto the detector, whereby to generate first data on the coordinate of each such point in the X direction, and, when the detector is bidimensional, also in the Y direction.
The system also employs a mask having a pair of apertures spaced apart from each other in the X direction for forming discrete images on the detector of each of the points. By virtue of the spacing between such images, they provide second data on the coordinate of each point in the Z direction. A computer scans the pixels to extract the first and second data for all the points and is hence able to determine the X, Y and Z coordinates of each point.
One of the limitations of this prior system is its inability to work accurately at or near focus, i.e. with values of the Z coordinate close to zero.
Another of the limitations of this system is that, in order to avoid sign ambiguity, i.e. to distinguish between positive and negative values of a Z coordinate, it is necessary to apply a bias (using a biprism or axicon lens), as explained in connection with FIG. 4 of the prior patent.